1. Field of the Invention
The present invention relates to a process for producing a photochromic cured product having excellent photochromism which is polymerized and cured in a short period of time by combining photopolymerization and thermopolymerization.
2. Prior Art
Photochromism which has attracted much attention for the past few years is a reversible phenomenon that a certain compound changes its color immediately upon exposure to light including ultraviolet rays such as sunlight or light from a mercury lamp and returns to its original color when it is placed in the dark by stopping irradiation. A compound having this property is called “photochromic compound”. Various compounds have been synthesized to such a compound heretofore but any common structure is not observed in these compounds.
Cured products which exhibit photochromism are obtained by coating the surface of a molded polymer with a photochromic compound or by dissolving a photochromic compound in a radically polymerizable monomer and polymerizing and curing the resulting mixture (to be referred to as “kneading method” hereinafter).
There are known methods for polymerizing a radically polymerizable monomer: (1) one in which polymerization is carried out by heat and (2) one in which polymerization is carried out by light. When a radically polymerizable monomer is to be polymerized by light in the above kneading method, the radically polymerizable monomer cannot be suitably polymerized because the contained photochromic compound absorbs ultraviolet rays required for the cleavage of an ultraviolet polymerization initiator and develops color itself by the absorption of ultraviolet rays to prevent the transmission of light.
As means of solving the above problems, JP-A 7-292011 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for carrying out polymerization by reducing the amount of a photochromic compound. However, the colorablility of the obtained photochromic cured product cannot be increased by this method. WO 96/37573 discloses a method which combines a specific ultraviolet polymerization initiator and activation energy rays having a specific emission spectrum. However, it cannot be said that a photochromic cured product obtained by this method is satisfactory in terms of photochromism durability.
The inventors of the present invention have found that photochromism durability greatly lowers in a polymer which is not polymerized completely by using photopolymerization. To obtain a perfect polymer, the exposure time must be prolonged. However, when the exposure time is long, a photochromic compound is deteriorated by exposure.
Consequently, when a photochromic cured product is to be obtained, thermopolymerization is generally used to polymerize a radically polymerizable monomer. However, the time required for thermopolymerization is extremely long, for example, several tens of hours, compared with photopolymerization. Therefore, thermopolymerization is unsatisfactory in terms of the productivity of cured products.